Terminal web

Abstract: The tight junction (zonula occludens), a belt-like region of contact between cells of polarized epithelia, serves as a selective barrier to small molecules and as a total barrier to large molecules1,2, and is involved in the separation between lumenal and basolateral compartments of the epithelium3,4. In the electron microscope, tight junctions show focal regions of apparent fusion between the adjoining cell membranes, and freeze-fractured membranes display an elaborate network of branching and anastomosing strands1,5–8. Very little is known about the molecular composition and architecture of tight junctions. The first specific zonula occludens-associated protein, designated ZO-1, has recently been identified in mammalian epithelial and endothelial cells9. Here we describe the identification and purification of a new component of this junc-tional complex in avian brush-border cells, which we name cingulin. Cingulin is an acidic, heat-stable protein, with a highly elongated shape. Immunofluorescence and immunoelectron microscopy of brush-border cells with anti-cingulin antibodies show that cingulin is localized in the apical zone of the terminal web, at the endofacial surfaces of the zonula occludens.

Abstract: Terminal webs prepared from mouse intestinal epithelial cells were examined by the quick-freeze, deep-etch, and rotary-replication method. The microvilli of these cells contain actin filaments that extend into the terminal web in compact bundles. Within the terminal web these bundles remain compact; few filaments are separated from the bundles and fewer still bend towards the lateral margins of the cell. Decoration with subfragment 1 (S1) of myosin confirmed that relatively few actin filaments travel horizontally in the web. Instead, between actin bundles there are complicated networks of the fibrils. Here we present two lines of evidence which suggest that myosin is one of the major cross-linkers in the terminal web. First, when brush borders are exposed to 1 mM ATP in 0.3 M KCl, they lose their normal ability to bind antimyosin antibodies as judged by immunofluorescence, and they lose the thin fibrils normally found in deep-etch replicas. Correspondingly, myosin is released into the supernatant as judged by SDS gel electrophoresis. Second, electron microscope immunocytochemistry with antimyosin antibodies followed by ferritin-conjugated second antibodies leads to ferritin deposition mainly on the fibrils at the basal part of rootlets. Deep-etching also reveals that the actin filament bundles are connected to intermediate filaments by another population of cross-linkers that are not extracted by ATP in 0.3 M KCl. From these results we conclude that myosin in the intestinal cell may not only be involved in a short range sliding-filament type of motility, but may also play a purely structural role as a long range cross-linker between microvillar rootlets.

Abstract: This paper presents a brief review of the ultrastructure of some of the structural elaborations of the plasma membrane that may be regarded as adaptations for specific cell functions. Among the relatively stable specializations of the free surface, it considers the striated border of the intestinal mucosa, the brush border in the nephron, and filamentous coatings on the membranes of certain cells of the gastric mucosa. It then turns to those transient configurations of cell surfaces involved in the dynamic processes of pinocytosis, phagocytosis, and liberation of secretory products and considers the turnover of membranes associated with these phenomena. Discussion of specializations of the contact surfaces includes the desmosomes and terminal bars and the present status of intercellular cement and interfacial canals. A section on specializations of the cell base considers the basement membrane and basal infoldings of the plasma membrane in cells engaged in active transport. And finally, there is a description of the terminal web and the marginal band of nucleated erythrocytes—specializations of the superficial cytoplasm concerned with maintenance of cell shape.